For safety consideration, in many or most power supplies for LED lighting devices and other switching mode power supplies the input is isolated from the output by a high frequency isolation transformer. This isolation transformer receives on its primary side high frequency (e.g., 40-100 kHz) AC electrical power, for example AC electrical power generated by an inverter, and outputs the AC electrical power to a rectifier on its secondary side. A low-ripple DC output from the rectifier is obtained by providing a filter or smoothing capacitor across the rectifier's output.
Stray capacitance exists between the primary and secondary windings of an isolation transformer. This stray capacitance is distributed in a way that depends on the isolation transformer construction (e.g., how the isolation transformer is wound).
In some cases, this stray capacitance can produce a charge pump effect which leads to an undesired increase in the output voltage of the rectifier when the output of the rectifier is unloaded or lightly loaded.
This high output voltage caused by this charge pump effect may be a violation of maximum output voltage specification for the power supply. Furthermore, when one of the windings (primary, secondary, or other auxiliary windings) of the isolation transformer is used to sense the output voltage, the sensed peak voltage will no longer be proportional to the actual output voltage since the voltage across the secondary winding is less than the output voltage due to the charge pump effect.
A traditional solution to this problem is using a dummy load at the output (i.e., output of the rectifier) which is large enough so that voltage drop across this dummy load caused by the charge pump current is less than the required no-load (open circuit) output voltage. This approach, however, causes a lot of wasted power dissipation and can impose substantial thermal stress on the dummy load.
Thus, there is a need in the art to provide compensation for an increase in an output voltage across the output of a power supply for a lighting device (e.g., an LED lighting device) or other switching mode power supply due to a charge pump effect of a parasitic capacitance of the isolation transformer.